You and your friend buy a NAS each. You are able to set up synchonizing between the two without too much trouble.

Setting up rsync with or without a guide is too much trouble. Naming your NASes and telling the admin interface of each NAS to sync to the other NAS is more like it.

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Long:

Many NASes have rsync capabilities. Setting up rsync is not done by your grand parents. It also means one must open a hole in the firewall which said grand parents are not comfortable with. That was many words for too-much-trouble.

Instead say you buy a Netgear NAS and send your NAS’s name to Netgear. Your friend does the same with his. Now each NAS can call Netgear through your firewall and Netgear can orchestrate the connection. I still haven’t figured out how the NASes go from talking to Netgear to each other to not wasting band width at Netgear’s. Is this solved with bittorrent?